This longitudinal study proposes to accumulate data to establish distance and velocity curves for growth of healthy subjects weighing less than 2500 grams at birth, born at varying ages of gestation. 275 subjects have been enrolled. There are 202 single born Caucasian subjects. The current age range is 7 to 12 years. Trained examiners using standardized techniques are recording the anthropometric measurements of weight, crown-heel length, crown-rump length and head circumference at the ages indicated in the protocol. From this data increments of growth are being calculated. During the first year of life, x-rays of the left lower extremity are used for measuring the length of the fibula and for studying the centers of ossification at the knee. The bone age is being determined from x-rays of the left wrist at 3, 6 and 8 years of age. The physical examination, motor and mental development are being recorded. Psychological testing was done at 4 years of age. All data will be subjected to statistical analyses.